1. Field of the Invention
The present invention generally relates to an improved method for fabricating a thallium-doped gradient index (GRIN) lens. Specifically, the invention is a method whereby a preform composed of a simplified glass composition is chemically and mechanically processed forming a GRIN lens with a parabolical-shaped refractive index.
2. Background
A gradient index (GRIN) lens is an optical component, commonly found in optical telecommunication systems, having an axially, a radially, or a spherically varying refractive index. The profile of the refractive index is tailored during fabrication to facilitate such applications as collimation, coupling, focusing, imaging and alignment.
For example, a parabolic-shaped refractive index provides both imaging and focusing capabilities. The profile of a typical parabolic-shaped refractive index is functionally described by the equation
 N2(r)=N2(0)[1−(gr)2+h4(gr)4+h6(gr)6+ . . . ]
where N0 is the refractive index along the central axis of the lens, r is the radial distance from the central axis, gr is a constant, and h is a high-order coefficient. The focal diameter varies between 0.7 and 2 micrometers for most GRIN lenses. The numerical aperture, NA, of a GRIN lens is calculated by the equation(2N0*ΔN)1/2where ΔN is the difference in refractive index between central axis and periphery along the lens and N0 is the refractive index along the central axis. Commonly, a GRIN lens will have a numerical aperture between 0.46 and 0.6.
The glass composition of a GRIN lens determines its functional and physical characteristics. A high-quality lens has a refractive index of at least 1.600, a chromatic transmittance greater than 90% for light with a wavelength between 380 and 2000 nanometers, and a high degree of composition uniformity so to minimize internal stresses, devitrification, phase separation, and corrosion.
The related arts describe refining agents, typically Cs, Ti, Al, As, Sn, Mg, Ba, Bi, Ge, Pb, Zr, Nd and metal oxides thereof, added to a glass composition to tailor optical properties, minimize devitrification, improve durability, improve weatherability, and adjust melt temperature and viscosity. However, such agents increase fabrication complexity and adversely alter both color and light absorption characteristics of the lens.
A typical cylindrical-shaped GRIN lens has a high refractive index along its central axis with a gradually decreasing index outward to the circumference. The refractive-index gradient is altered by varying the glass composition.
The preferred method within the art for varying glass composition is referred to as diffusion or ion exchange. A thallium-doped glass cylinder having a uniform, high-refractive index is soaked within a molten alkali salt bath, one example being potassium nitrate (KNO3), thereby allowing for an exchange between thallium ions in the glass composition and potassium ions in the bath. The exchange produces a thallium ion gradient within the glass composition and a corresponding refractive index profile. The ion exchange process is influenced by composition uniformity and adversely influenced by stresses, imperfections, and contaminants within the composition. Furthermore, the refining agents described above adversely interact with alkali salts inhibiting the ion exchange process and causing physical degradation to the lens.
Therefore, what is currently required is a method providing for the fabrication of a high-quality GRIN lens composed of a simplified glass composition thereby avoiding the complexities and problems associated with refinement agents.